This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of thickened aqueous liquids for mobility control.
In the recovery of oil from oil-bearing reservoirs, it is usually possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
One difficulty often encountered in waterflooding operations is the relatively poor sweep efficiency of the aqueous displacing medium; that is, the injected displacing medium tends to channel through certain portions of the reservoir as it travels from the injection system to the production system and to bypass other portions. Such poor sweep efficiency or macroscopic displacement efficiency may be due to a number of factors such as differences in the mobilities of the injected displacing liquids and the displaced reservoir oil and permeability variations within the reservoir which encourage preferential flow through some portions of the reservoir at the expense of other portions.
Various techniques have been proposed in order to improve the sweep efficiency of the injected displacing medium and thus avoid premature breakthrough at one or more of the wells comprising the production system. The most widely used procedure involves the addition of thickening agents to the injected displacing medium in order to increase the viscosity thereof and thus decrease its mobility to a value equal to or less than the mobility of the displaced reservoir oil, resulting in a "mobility ratio" of oil to water which is less than or equal to one. Many polymeric thickening agents have been proposed for use in such mobility control applications. One well known class of polymers which may be employed for this purpose is the group of polysaccharides produced by the action of bacteria of the genus Xanthomonas on carbohydrates as disclosed in U.S. Pat. No. 3,305,016 to Lindblom et al. As disclosed in this patent, these polysaccharides generally are employed in concentrations within the range of about 0.005-1.0 weight percent, with concentrations within the range of about 0.05-0.25 weight percent normally being preferred. Among the advantages attributed to these polysaccharides by Lindblom et al., are their usefulness in the presence of sodium chloride and other salts and their thermal stability and resistance to adsorption onto rock surfaces within a subterranean reservoir.
While Lindblom et al. refer to the thermal stability of the polysaccharides in the context of reservoir temperatures generally ranging between about 85.degree. to 150.degree. F., these polysaccharides are subject to thermal degradation which limits their application in relatively high temperature reservoirs. Waterflood oil recovery processes normally involve considerable lengths of time, on the order of months or several years, before recovery of oil from the formation is completed. Over such time intervals, the Xanthomonas polysaccharides such as disclosed in Lindblom et al. undergo significant thermal degradation at temperatures greater than about 60.degree. C. (140.degree. F.). Temperatures in excess of 60.degree. C. are normally encountered in subterranean oil reservoirs at depths of about 4,000 feet or more.
As disclosed in U.S. Pat. No. 3,532,166 to Williams, Xanthomonas polysaccharides are also subject to adsorption onto reservoir rock surfaces. As disclosed in Williams, such adsorption may be retarded through the use of inorganic sacrificial agents such as water-soluble phosphates and/or water-soluble carbonates. Suitable water-soluble carbonates include the alkali metal carbonates such as sodium carbonate. The sacrificial agents may be employed in an aqueous solution injected prior to the polysaccharide solution or they may be added to the thickened polysaccharide solution.
The injected aqueous medium employed in waterflooding may contain various other additives in addition to the polymeric thickening agents. For example, the injected water may contain surface-active agents which reduce the interfacial tension between the injected water and oil or it may contain various agents such as alkaline agents which react with injected or indigenous material in the reservoir oil to produce surface-active agents within the reservoirs. Thus, U.S. Pat. No. 3,637,017 to Gale et al. discloses a waterflood oil recovery process employing various combinations of alcohols and surfactants. Surfactants disclosed for use in the Gale et al. process include petroleum sulfonates, and alcohols disclosed by the patentees include aliphatic alcohols having from 1 to 8 carbon atoms. The surfactant and alcohol may be injected simultaneously in an aqueous solution or the surfactant solution may be injected first and then followed by an aqueous solution of alcohol. Thickening agents may also be employed in the Gale et al. process. Thus, as disclosed in column 9 of Gale et al., a 2 percent solution of petroleum sulfonate containing also 0.75 weight percent sodium carbonate, 0.5 percent ammonia, and sufficient heteropolysaccharide of the type disclosed in the aforementioned Lindblom et al. patent to raise the viscosity of the surfactant solution to approximately 10 centipoises is injected. This surfactant solution is then displaced by water containing 2 percent by volume isobutanol and sufficient heteropolysaccharide to increase the viscosity of the solution to about 10 centipoises.